The primary objective of the proposed research is to determine the molecular basis for resistance and susceptibility to larval schistosomes in genetically-defined strains of Biomphalaria glabrata. Briefly, in vivo cultivation methods in conjunction with membrane probe and radiolabeling techniques will be employed to: (1) Investigate antigen sharing or antigenic masking by early schistosome stages (miracidia and mother sporocysts) as possible mechanisms for evasion of host recognition; (2) Compare the surface membrane components of susceptible and refractory snail hemocytes, and to determine if either strain possesses schistosome-reactive hemocyte receptors (recognition mechanism); (3) Investigate the possible role of hemocytes in the transference of schistosome "immunity" from refractory to susceptible snail hosts (adoptive transfer); (4) Study the effects of larval schistosomes on hemocyte proliferation in susceptible and refractory snails as a possible immune interference mechanism.